In recent years, wavelength conversion elements using ferroelectric single crystals having an excellent nonlinear optical constant and an electro-optic constant are being intensively studied. Among them, development of wavelength conversion elements adopting a quasi-phase-matched system is remarkable, due to improvements of the manufacturing technique of ferroelectric single crystals with high quality and the domain inversion structure formation technique.
Wavelength conversion elements tunable in a wide range of wavelength (hereinafter denoted merely by tunability) utilizing lithium niobate single crystals have been developed (see for example, non-patent literature 1).
FIG. 16 is a schematic diagram showing a multi-grating type quasi-phase-matched (QPM) parametric oscillator (OPO) by the prior art.
QPM OPO 1600 comprises a wavelength conversion element 1601, a first mirror 1602, a second mirror 1603, and a moving means 1604.
The wavelength conversion element 1601 is made of a congruent lithium niobate (CLN) wafer. The wavelength conversion element 1601 is 0.5 mm in thickness in the direction parallel to the polarization direction of the CLN wafer, and length of the element L is 26 mm.
The wavelength conversion element 1601 has plural domain inversion structures (gratings) with different periods. The width of each grating, WG, is 500 μm. The width of the space between each of gratings is 50 μm. The period of each grating is from 26 μm to 36 μm. The gratings are arranged so that the periods of gratings increase with an increment of 0.25 μm.
These gratings are produced by the lithography technique and electric field poling process repeatedly by using a mask having a predetermined period for each grating. In the figure, only a part of multi-gratings is illustrated.
Each of the first mirror 1602 and the second mirror 1603 has a curvature radius of 150 mm. The first mirror 1602 and the second mirror 1603 are placed through the wavelength conversion element 1601, and the distance between the mirrors is 30 mm.
The moving means 1604 moves the wavelength conversion element 1601 in the parallel direction.
Next, the operation of this QPM OPO 1600 is explained.
The pump laser light (the first wavelength λ1=1.064 μm) generated from a Q-switch Nd:YAG laser (not shown) is incident, through the first mirror 1602 with predetermined beam diameter, on the grating with the predetermined period in the wavelength conversion element 1601.
At that time, the moving means 1604 displaces beforehand the wavelength conversion element 1601 so that the pump laser light is incident on the predetermined grating.
Then, the pump laser light having the first wavelength λ1 is converted into signal light having the second wavelength λ2 and idler light having the third wavelength λ3, dependent on the period of the wavelength conversion element 1601.
At this time, the signal light centering on the wavelength of 1.54 μm is reflected in part by the first mirror 1602 and the second mirror 1603. The signal light and the idler light transmitted through the second mirror 1603 without being reflected thereby, exit the QPM OPO 1600.
Using the QPM OPO 1600 designed in this way, when the first wavelength λ1 of the pump laser light is 1.064 μm and the periods of the multi-gratings are from 26 μm to 32 μm, it can be achieved only by displacing the wavelength conversion element 1601 using the moving means 1604, that the second wavelength λ2 of the signal light has a wavelength of variable range from 1.36 μm to 1.98 μm and the third wavelength λ3 of the idler light has a wavelength of variable range from 4.83 μm to 2.30 μm.
Also, a wavelength conversion element using a lithium tantalate single crystal with a stoichiometric composition has been developed (for example, see patent literature 1).
FIG. 17 is a diagram showing a wavelength conversion system according to a prior art.
The wavelength conversion system 1700 comprises a variable wavelength laser 1701, a lens 1702 and a wavelength conversion element 1703.
The wavelength conversion element 1703 consists of a lithium tantalate single crystal with nearly stoichiometric composition having 0.3 mm to 5 mm in thickness. The wavelength conversion element 1700 has periodically poled structures with the period from 3 μm to 5 μm produced by the electric field poling process.
In this wavelength conversion system 1700, the light emitted from the variable wavelength laser 1701 (the fundamental wave) is incident on the wavelength conversion element 1703 through the lens 1702. The fundamental wave incident on the wavelength conversion element 1703 phase-matches (quasi-phase-matches) with the second harmonic of the fundamental wave by periodically poled structures in the wave-traveling direction of the light. Thus, the fundamental wave is converted into the second harmonic during propagating through the wavelength conversion element 1703.
Non-patent literature 1; L. E. Meyrs, et al., Multigrating quasi-phase-matched optical parametric oscillator in periodically poled LiNbO3, OPTICS LETTERS, Apr. 15, 1996, Vol. 21, No. 8, pp. 591-593.
Patent literature 1; Japanese Patent Laid-Open No. 2002-90785.